A frequent requirement in the copying, duplicating and printing industry is the reproduction of a continuous tone image, such as that produced through photography, on a printer which is incapable of reproducing gray colors. Newspaper printers and office copiers are the most common examples. A reasonable reproduction of gray scale originals can be accomplished through the process of halftoning. In this case, the copy comprises a matrix of black and white dots. The black dots are increased in size relative to the white dots to represent areas of darker gray while the size of the white dots are relatively increased to represent lighter shades of gray.
A more difficult problem occurs where the original itself is a halftone representation, rather than a photograph or equivalent. The production of a halftone copy from a halftone original usually results in a degraded copy since the original screen generates a large amount of non-informational content which, consequently, reacts with the copying process to produce Moire patterns, irregular shape and size of dots and a loss of texture and gray scale.
The most common method of improving the quality of reproduction from a halftone original is to filter out those screen frequencies that are produced by the halftoning process. The apparatus in raster scanning systems typically comprises a raster input scanner for converting the halftone original into an analog voltage, an analog filter to attenuate the high frequencies of the electrical signal, a halftone screener to convert the analog voltage into a halftone representation, and a printing engine to produce the hard copy. One problem with this system is that the filtering out of screen frequencies by a low pass filter also filters out most of the highlights and sharp edges that would otherwise have been reproduced in the copy. The result is a halftone reproduction that has an unfocused or washed-out look.
An alternative system converts the raster input scanner analog voltage into a digital representation, typically six bits or more per pixel where a pixel is defined as the smallest image (picture) element discernable by the system. For example, in a raster input scanner using a CCD array, the smallest picture element, pixel, is the analog voltage output of one CCD cell. This voltage subsequently can be converted to a digital gray scale representation requiring a plurality of bits per pixel, or either analog or digital representation can be compared to an analog or digital reference voltage to produce a one bit pixel. If a digital gray scale data stream is produced it may be operated on by some type of two dimensional digital filter to filter out the halftone screen frequencies. This process is time consuming and requires a relatively large amount of hardware since the digital representation of each pixel is typically multiplied by a weighting factor as part of the filtering process. The digital realization of low pass filtering also results in the loss of highlights and sharp edges.
What is required by the industry is a high speed method of reproducing halftone images which may be implemented in a relatively simple circuit and which would maintain a reasonable amount of detail in the copy.